1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to arc fault circuit interrupters. The invention also relates to methods of detecting arcing conditions.
2. Background Information
Maintenance personnel often need to work on low voltage electric power circuits that are energized, for instance, when testing circuit breakers. The circuit breakers have trip functions designed to protect the power circuits from overloads and overcurrents. These trip functions typically include a delayed trip function that allows downstream circuit breakers closer to the fault to respond first in order to limit interruption of service in that circuit only. The delayed trip function also permits temporary overloads, such as those associated with the starting current of relatively large motors.
The coordination between circuit breakers can be enhanced by zone interlocking in which a breaker lower in the protection hierarchy sends a signal upon detecting a fault to an upstream breaker to temporarily block operation of the latter breaker giving the downstream breaker time to respond. Absence of a zone interlock signal from the downstream breaker indicates that the fault is above that breaker, thereby allowing the upstream breaker to respond relatively more quickly than if its operation were temporarily blocked.
Often, a power circuit breaker will also have an instantaneous trip function that responds, without delay, to relatively very high currents, such as those associated with a short circuit. These circuit breakers can also have a ground fault trip function that typically also incorporates a delay time.
Maintenance and test personnel can be exposed to live terminals when working or making measurements on energized switchgear. The potential for arc flash resulting from an accidental fault requires that such personnel wear protective clothing.
U.S. Pat. No. 7,203,040 discloses an arc reduction maintenance switch (ARMS) for reduction of arc flash energy and the severity of arc flash exposure. Specific trip functions are manually overridden with a maintenance trip function that reduces arc energy should a fault occur. The manually enabled and disabled maintenance trip function reduces the pickup currents of the specified trip functions and/or reduces or eliminates the time delays of the specified trip functions. For example, instantaneous trip is reduced from 10 times to 2 times rated current, short delay pickup is reduced from 8 times to 1.5 times rated current, the short time delay of 0.5 second is reduced to 0.050 second (essentially no time delay), the (equipment protection) ground fault pickup current is reduced from 0.5 times to 0.25 times rated current, and the ground fault time delay is reduced from 0.5 second to 0.050 second (essentially no time delay). As another example, instantaneous trip is reduced from 10 times to 4 times rated current, short delay pickup is reduced from 8 times to 3 times rated current, the short time delay of 0.5 second is reduced to 0.050 second (essentially no time delay), the (equipment protection) ground fault pickup current is reduced from 0.5 times to 0.25 times rated current, and the ground fault time delay is reduced from 0.5 second to 0.050 second (essentially no time delay).
The ARMS reduces arc flash energy by lowering the trip level of a fault and is limited to fault levels higher than the frame rating of a molded case circuit breaker. The ARMS does not detect arcing conditions at fault levels lower than the frame rating. Also, it needs to be armed, when required, and disarmed after service is completed.
Ground faults (such as arc flashes) or most parallel arc faults are short circuit events, which conventional industrial circuit breakers can detect and interrupt. However, such circuit breakers may trip too slowly due to the breaker withstand requirement in some cases. That is why the ARMS feature needs to be armed or the trip level needs to be lowered, while service is being performed, in order that the circuit breaker can trip faster and, therefore, interrupt the fault faster.
In industrial applications, conventional arc fault circuit interrupters (AFCls) may trip too quickly for “arcing” arising during conventional circuit breaker switching, which may last up to about a full cycle or so.
There is room for improvement in arc fault circuit interrupters.
There is also room for improvement in methods of detecting arcing conditions.